1. Field of the Invention
The present invention generally relates to the manufacture of electrical circuits and more particularly to the manufacture of multi-layer ceramic interconnect structures.
2. General Description of Background
In the manufacture of multi-layer ceramic (MLC) interconnect structures conductive paste is injected through a nozzle into a screening mask. The mask extrudes a thick film conductive pattern onto a greensheet to form a circuit. The paste flows through holes in the surface of the mask to fill lines and other electrical structures in the mask and flows into perforations in the greensheet to provide electrical connections between layers of the MLC structure. At the same time, air flows out of the volume of the mask to make room for the paste. Some of the paste also flows back out onto the surface of the mask. This application is concerned with the interface between the nozzle and the mask as the paste flows from the nozzle.
Manufacture of multi-layer ceramic (MLC) interconnection structures is described in U.S. Pat. No. 4,245,273, to Feinberg et al., for PACKAGE FOR MOUNTING AND INTERCONNECTING A PLURALITY OF LARGE SCALE INTEGRATED SEMICONDUCTOR DEVICES, assigned to the assignee to the present invention, and hereby incorporated by reference. In these structures, a potentially differing interconnection pattern is formed on each of a multiplicity of layers of ceramic substrate. These intricate interconnection patterns include vias formed by perforating greensheet substrates and selectively filling the perforations with conductive paste to provide electrical continuity between the ceramic substrate layers of the MLC structure.
The respective greensheet substrates are then stacked and sintered under pressure and high temperature to form a unitary ceramic structure with many embedded interconnection lamina to provide electronic interconnection structures of high connection complexity. However, testing of each layer as it is formed requires a number of steps at least corresponding to the number of layers and an equal number of geometrical setups to allow testing by automated machinery. For this reason, it is common to optically inspect each layer as it is formed and to test only a portion of each layer for circuit continuity to some degree of confidence. Full electrical continuity is only tested a single time after the entire multi-layer ceramic (MLC) structure is assembled and sintered. This final test would be required even if the individual layers were fully tested individually, since the electrical connection patterns can be damaged by either the assembly or the sintering steps of the process.
Therefore, to achieve reasonably high manufacturing yields it is particularly necessary to form the individual layers by techniques which result in an extremely low incidence of defects. The conductive patterns must also be formed with high regularity and consistency to reduce susceptibility to the formation of defects during assembly and sintering.
The formation of the conductive patterns and filling of perforations to form vias in the individual substrate layers is done by assembling a stencil known as a mask on a greensheet substrate. The mask contains a plurality of channels for extruding the pattern onto the greensheet and filling the via perforations. Conductive paste is injected into the mask from the discharge end of a nozzle which is elongate across the width of the mask with the approximate same length as the width of the mask.
The mask directs the paste from the elongate nozzle to fill the via perforations and extrude the conductive paste circuit onto the greensheet as the nozzle is moved laterally to traverse the length of the mask. The nozzle may be traversed back and forth along the length of the mask in multiple passes as required. Then the nozzle and mask are separated from the greensheet leaving the conductive pattern on the greensheet. This process is commonly known as screening. Once a patterned greensheet layer is initially formed, other processing steps, such as drying, may also be employed to stabilize the pattern for assembly. Then the layers are assembled and, typically, sintered, as indicated above.
The use of a mask and the fineness of typical interconnection patterns and vias requires high paste pressure to be developed in the nozzle for the application of the conductive paste. At one time, the layer of conductive paste was typically applied by means of a Teflon.TM. squeegee. As conductive patterns increased in complexity, however, a squeegee was not able to develop sufficient pressures to reliably penetrate the masks with the conductive paste to form the desired interconnection patterns and to reliably fill the via perforations in the greensheets. Accordingly, it is presently deemed desirable to apply the conductive paste with a nozzle which confines the conductive paste against the mask and greensheet and allows full penetration of the mask and greensheet perforations. A description of a screen printing method for forming MLC structures using a nozzle is described in U.S. Pat. No. 4,808,435, to Cropp et al., assigned to the Assignee of the present invention and which is hereby incorporated by reference.
The cost of mask and nozzle maintenance including down time and replacement costs are critical to the economy of screening. The useful life of the masks and nozzles are measured by the number of passes the nozzle makes across the mask before the mask must be replaced. These measurements of mask and nozzle life are known in the art as pass factors.
High forces are applied by a mechanical spring, piston or other force transducer unit to the nozzle in the direction perpendicular to the plane of the mask (normal to the mask) in order to overcome the high pressure of injection of the paste and in order to confine the paste to maintain the paste pressure by insuring contact between the nozzle and the mask. Typically the normal force applied to the nozzle is preset at a fixed level to hold the nozzle against the mask for the highest expected working pressures. This results in relatively high nozzle forces against the mask. The force of the nozzle against the mask results in sliding friction forces as the nozzle traverses the mask during extrusion.
The normal and friction forces applied by the nozzle to the mask result in high stresses in the screening mask especially in the tabs which bridge between sections of the mask and across conductive lines in the pattern in order to hold the mask together. These stresses subject the mask to distortion and potential damage and contribute to broken tabs, dents and scoring of the mask. All these failure modes are fatal to the operation of the screening mask and the quality of the vias and circuit pattern produced. Also particles on the mask tend to wedge between the discharge end of the nozzle and the mask which dents and scores the mask and/or nozzle and places high loads on the tabs.
The masks are typically 2 to 3 mils thick and tabs are typically only about one mil thick and only slightly wider to avoid leaving opens in the mask separations across which they bridge. Efforts by those skilled in the art to prevent mask tabs from breaking have centered around increasing the physical size of the individual tabs and reducing the distance between tabs. There has been some reduction in tab failures but the rate of premature mask failure due to tab breaking and other damage has not been acceptable.
The sliding friction results in wear of the mask, which is typically formed of a Molybdenum, Nickel or a Ni-Cu-Ni sandwiched sheet. The friction also causes wear of the nozzle and, for that reason, great effort has been expended to provide a nozzle surface which is resistant to abrasion. A nozzle formed of Tungsten Carbide is disclosed in SCREENING NOZZLE WITH NON-LINE CONTACT by R. C. Brilla et al. in IBM Technical Bulletin, Vol. 25, No. 6, November 1982, which is also incorporated by reference. An improved nozzle utilizing replaceable carbide rods for the nozzle surface is disclosed in co-pending application Ser. No. 07/631,540 CARBIDE ROD SCREENING NOZZLES by Andris et al. filed Dec. 21, 1990, incorporated herein by reference.
Where the mask has coarse features relative to the width of the contact surface between the nozzle and the mask, paste leaks around the contact surfaces of the nozzle at these features. This potential problem can be solved in the manner disclosed in U.S. Pat. No. 3,384,931, to Cochran et al., assigned to the assignee of the present invention and also incorporated by reference, by using a more complex stencil structure. That patent discloses a mask with features formed only on the side facing the greensheet, and with many tiny apertures in the opposite side facing the nozzle to allow penetration of the paste from the nozzle to the features and escape of air from the features. In that method, line contact between the elongate nozzle and the mask is preferred. Line contact allows a narrow pressure angle to be formed by essentially tangent contact between the nozzle and the mask in order to increase effective screening pressure. The viscosity of the conductive paste prevents high flow rates through the via perforations and through the pattern in the mask in order to maintain the pressure of the paste in the nozzle as the mask features are filled. The low viscosity of air allows the air to escape through the vias and mask features/pattern as the nozzle is dragged across the mask.
The more complex mask structure, however, makes it especially desirable that wear and scoring of the mask be minimized. To a certain degree, mask degradation is reduced by the line contact of the nozzle. However, as the nozzle itself becomes worn or otherwise degraded due to both friction with the mask and interaction with the conductive paste, the nozzle profile becomes more flat and the contact area becomes wide, increasing the tendency for scoring of the mask to occur.
The short useful life of the masks as well as the complexity thereof significantly increases the cost of the screening process and degrades the quality of the screened pattern. In addition, the substrate or carrier can become contaminated by metal shavings resulting from the scoring. Scoring also degrades the screening process since it degrades the tightness with which the nozzle fits against the mask, causing smearing of the paste as well as potential loss of pressure.